The self-aligning roller bearing has the advantage of capable of preventing the generation of abnormal load and increasing the radial load capacity because a contact condition of the rolling elements is not varied even when the outer ring or the inner ring is inclined due to a fitting error or an impact load. For this reason, the self-aligning roller bearing is used widely as various roll neck bearings in the papermaking machine, the vehicle bearing, various industrial bearings, and so on.
By the way, the normal ball bearing or cylindrical roller bearing is broken down by the subsurface initiated facture in a clean environment in which an oil film is satisfactorily formed. This subsurface initiated facture signifies that the fatigue crack is generated and widened from the non-metallic inclusion contained in the material as the starting point. Therefore, lifetime enhancement of the ball bearing and the cylindrical roller bearing can be achieved by increasing an index of cleanliness of material.
However, unlike the above bearings, in some cases the surface initiated failure is caused in the self-aligning roller bearing according to the application condition. This surface initiated failure signifies that the minute plastic flow is generated on the surface of the inner ring in the clean environment and then the peeling crack is generated and spread from there to lead to the flaking. As a result, an increase in the index of cleanliness of material does not have a noticeable effect on the lifetime enhancement of the self-aligning roller bearing.
In the self-aligning roller bearing, a skew largely affects prevention of the heat generation or lifetime enhancement of the bearing.
As the measure for this, in order to control a coefficient of friction between the inner ring and the rolling elements and a coefficient of friction between the outer ring and the rolling elements, it is applied to control a contact area of the bearing and a surface roughness of the raceway surface. For example, such an instance is disclosed that the skew is controlled by setting the surface roughness of the outer ring raceway surface larger than the surface roughness of the inner ring raceway surface (the surface roughness of the inner ring raceway surface is 0.1 μmRa or less, and the surface roughness of the outer ring raceway surface is 0.2 μmRa or more) to attain the lifetime enhancement (see JP-B-57-61933).
As the reason for lifetime enhancement, such a reason is assumed that, in case the roughness of the outer ring is made larger than that of the inner ring, a positive skew to incline the rolling elements to the outside of the bearing occurs to reduce the axial load. Also, it is concluded that, since a negative skew to incline the rolling elements to the inside of the bearing increases the axial load, such negative skew exerts a bad influence upon the lifetime of the bearing.
However, when the roughness of the outer ring raceway surface is made simply larger than that of the inner ring raceway surface, in some cases either the negative skew is ready to occur or the lifetime is prolonged even though the negative skew is increased. Therefore, it is hard to say that the skew control has a critical effect on an extension of the lifetime. Also, in case the roughness of the outer ring raceway surface is made excessively large, unevenness of the outer ring raceway surface becomes larger than a thickness of the oil film. Therefore, the lubrication condition is worsened and conversely the lifetime of the bearing is liable to become short.
Also, the approach of increasing a frictional force by forming the surface roughness of the outer ring raceway surface larger than that of the inner ring raceway surface is effective for the skew control or the suppression of minute slip. However, actually one bearing has a variation in the surface roughness of the inner ring raceway surface and the outer ring raceway surface (variation in the circumferential direction) according to locations of the inner and outer rings. Therefore, magnitudes of the frictional forces generated at the contact portion between the inner ring raceway surface and the rolling element and the contact portion between the outer ring raceway surface and the rolling element are varied according to the rotation of the bearing in respective locations. As a result, it is likely that the actual effect of suppressing the skew and the slip in the minute area is varied.
For example, when the roughness was measured at several locations in one bearing, individual values of the roughness are varied. In the situation that the machining condition is not good when viewed in the light of the roughness range, if there exist the portions at which the surface roughness is reversed between the inner ring raceway surface and the outer ring raceway surface because the roughness range of the inner ring raceway surface and the roughness range of the outer ring raceway surface come very close to each other or overlap with each other, a frictional force generated at the contact portions between the rolling elements and the inner and outer rings is varied and then the rotation of the rollers becomes unstable. Thus, the minute slip is generated at the contact portions between the inner ring and the rolling elements, and also the surface damage such as the peeling, or the like is prone to occur.
Therefore, it is not enough just to make the surface roughness of the outer ring raceway surface larger than that of the inner ring raceway surface. As a consequence, a ratio of the surface roughness between the outer ring raceway surface and the inner ring raceway surface must be specified with regard to a roughness distribution of the outer ring raceway surface and a roughness distribution of the inner ring raceway surface in the bearing.
Also, in the self-aligning roller bearing, a retained austenite content is reduced substantially to 0% by applying the high temperature tempering to the inner and outer rings at 200° C. or more since normally the retained austenite is decomposed under the high temperature application condition to cause a dimensional change. This high temperature tempering process can cause the retained austenite to decompose, but such process exerts the harmful effect such that the hardness is lowered. Therefore, the surface damage such as the peeling, or the like is apt to occur on the inner ring and thus the lifetime of the bearing is shortened.
In contrast, if the surface roughness of the outer ring is increased, i.e., if an upper limit value of the roughness range of the inner ring raceway surface on a center line is set larger than a lower limit value of the roughness range of the outer ring raceway surface on a center line, the surface damage such as the peeling, or the like of the inner ring can be suppressed as described above, and also the facture of the inner ring can be suppressed. On the other hand, a frictional force generated at the contact portions between the outer ring whose roughness is large and the rollers whose roughness is relatively appropriate is increased and thus the outer ring drives the rollers. As a result, the fatigue of the roller surface makes progress considerably.